1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery using, as a positive electrode active material, a lithium-containing transition metal oxide containing nickel and manganese as transition metals.
2. Description of the Related Art
In recent years, to solve environmental issues of exhaust gases, HEV (Hybrid Electric Vehicle) employing an automotive gasoline engine and an electric motor in combination has been internationally developed. As an electric power source for HEV, conventionally nickel-hydrogen secondary batteries have been employed, however it is highly desired to develop Specifically usable lithium ion secondary batteries with high voltage and high capacity.
One of important matter for the lithium ion secondary batteries for HEV is cost down. In commercialized lithium ion secondary batteries for a power source of portable electronic appliances such as cellular phones, camcorders, note type personal computers, and the like, Co-containing compounded oxides have been used mainly as a positive electrode active material, however from a viewpoint of the cost, positive electrode materials containing no costly metal element such as Co are preferable for lithium ion secondary batteries for large scale HEV. Further, in use for HEV, in order to efficiently recover the battery capacity, it is more desirable as the charging side output is higher in terms of system planning. However, in the case of using the conventionally used active materials such as lithium cobalt oxide LiCoO2, lithium nickel oxide LiNiO2, and lithium manganese oxide LiMn2O4, since the positive electrode potential is high, there is a problem that battery voltage becomes high and the output in the charging side is lowered. Therefore, to be used for HEV, batteries having low charge-discharge voltage are required.
As a positive electrode active material for a lithium ion secondary battery for HEV satisfying the above-mentioned requirements, active materials containing only elements which are relatively economically supplied such as lithium-containing olivine type phosphates, Ni—Mn type compounded oxides have been widely investigated in these years. Among them, Li(LiNiMn) compounded oxides having a crystal structure belonging to space group R-3 m and containing Li in transition metal site have drawn attraction as an economical and high capacity positive electrode material since it causes dissociation of Li2O at 4.5 V or more (vs. Li/Li+) and gives capacity attributed to redox reaction of Mn3+/Mn4+ (US Patent Laid-Open No. 2003/0108793A1)
However, since batteries using these positive electrode active materials have high electric resistance as compared with batteries using Co type compounded oxides, in the case of charging and discharging at high electric current, the resistance over voltage is increased and the battery voltage is decreased to result in a problem that no sufficient output property can be obtained.